The objective of this study is to gain increased knowledge about the onset of acute myocardial infarction which, in turn, will lead to more effective preventive therapy. The impetus for the study is the recent documentation by the applicants that the onset of infarction is more frequent from 6 a.m. to noon than during other times of the day. This fact calls into questions the widely held- -but poorly supported--impression that the onset of myocardial infarction is a random event. Twenty-two coronary care units in Massachusetts have formed a study group to investigate the determinants of the timing of myocardial infarction onset using a case-control design. Over a 3.5 year period, detailed information about the events immediately prior to the onset of myocardial infarction will be collected from approximately 7,000 patients. Relative risks associated with hypothesized risk factors for onset will be estimated by comparing observed exposure frequencies immediately prior to myocardial infarction with expected values derived from exposure frequencies in the same patients at other times of the day and in other patients at the same time of the day. The following hypotheses will be tested: 1) Precipitants of myocardial infarction (at any time of day) such as assumption of he upright posture, intense physical activity, a large meal, and acute emotional stress, will be identified. 2) The morning peak in infarct incidence will be higher and narrower when onset-time is adjusted for the variable wake-times of the population. 3) The relative risk of morning onset will e reduced by beta-blockade and/or aspirin usage. 4) The relative risk of morning infarction will not be altered by gender, age or cigarette smoking. When the relationship of infarction onset to time of awakening is clarified, and possible precipitants evaluated, increased information about triggering processes will be available. Such information will assist in the design of preventive therapy which can then be evaluated in randomized trials.